Allow calls with known writes when trying to remove allocas [part 2]

This is a slight generalization of D115829. I noticed this while restructuring code for a follow up patch to perform the same optimizations in DSE.

If we have a call whose only visible effect is writing to an alloca, and we're removing the alloca anyways, we don't care if the call also reads from the same alloca. That read will be unobservable and thus doesn't block removal of the call.

Worth noting is that this observation generalizes for non-argument reads. It just happens that case reduces to a readonly call, and is already handled separately.

Differential Revision: https://reviews.llvm.org/D115898